Late surge in ASR cases not concern for soybean crop

Louisiana’s latest confirmation of Asian soybean rust came Sept. 25 with a find in St. Landry Parish. The disease has been found in 16 parishes.

“If you look where these cases have occurred on the map, there are only a couple of soybean parishes left that don’t officially have ASR,” says Clayton Hollier, LSU AgCenter plant pathologist “However, we have deep suspicions the disease is there. It just hasn’t been found yet. It’s probably just a matter of time before ASR is found in those holdouts.”

The late-season up-tick in ASR discoveries isn’t surprising, says David Lanclos. “Our weather conditions this year, overall, didn’t seem favorable for ASR development,” says the LSU AgCenter soybean specialist. “Things have changed to ASR’s benefit the last few weeks and that’s when the snowball started rolling. Right now, south of I-10 in sugarcane country, ASR is so bad in some spots it’s visually obvious.”

Once mired in drought, Louisiana over the last several months has had intermittent showers and high humidity. That ensured extended dew-leaf periods.

“Consider where ASR was first found in the state’s lower parishes and track it to the next find (midsummer), north of Alexandria,” says Hollier. “That tells me there was a lot of spore movement carried by wind. Spore showers can do strange things.”

The ASR spores could have been brought from outside the state, but Hollier believes it’s more likely current infections were caused by home-grown inoculum.

“We’ve just about proven that by the many infected sites we’ve found and tracked. We first found it in kudzu. I think that’s where it overwintered (in 2005). From there, it waited for wind currents and better environmental conditions. Those didn’t line up until late in the season, thank goodness.”

The latest find in Iberia Parish — one of the parishes with a kudzu find early — is worrisome. The infection level reminds Hollier of that often seen in Brazil.

“If it had been found earlier in the season at this level I’d say it was dangerous to our (neighboring states). Every leaf is just about covered with rust. The good part is the crop was at R-7 when (ASR) hit and will soon be harvested. It’s a concern because the infection reached such a hot level. All other finds, at least initially, were low in severity: a few pustules on a few leaves in a few plants.”

ASR shifted to a more aggressive level once canopies closed. “The canopy maintains moisture longer. That’s true whether the moisture is from dew, rainfall, or irrigation.” With higher humidity, water is much more prevalent on lower leaves — something ASR needs for development.

“With open canopies, we tend to see less rust. The wind is blowing, air is circulating and plantings are drying.”

Will this be the common pattern — a few incidents that blow up late in the season — for ASR in Louisiana? “Much will be determined by the initial inoculum load. Do we have ASR surviving in kudzu in the deep South? Are those sites protected enough so they maintain some level of rust throughout the winter? If that’s the case, what we’re seeing might be common.”

Hollier says there are two factors to consider. “First, we had an extended drought this year. That delayed development of crops and even some planting. That extended dry period knocked back the chances of rust developing.

“Second, what will happen during the winter? If the winter is mild, as it was in 2005, the chances the ASR inoculum will survive are very good.”

With a more severe winter, as in 2004 when a freeze reached the Gulf of Mexico, the chance of ASR surviving is much less.

“We’ll be monitoring the (ASR kudzu) sites in southern Louisiana all winter. Even if freezing temperatures hit, we’ll continue to monitor those sites. We want to know if there’s any survival deep in the canopy.”

The good news is fungicides have proven effective against the disease. “Wherever we’ve sprayed with a tank-mix strobilurin, ASR has been controlled at least adequately,” says Lanclos. “I believe the reason we haven’t had a more rapid spread of the disease is that proper compounds were applied in a timely fashion.”